1. Field of of Invention
This invention relates to a sand filter for producing fluids through well bores from subsurface formations. It relates especially to a multi-layer sand screen for removing the sand before it is produced through the well bore.
2. Setting of the Invention
Oil and gas are produced from underground formations through well bores drilled from the surface to the formation. Some oil and gas are contained in the pores of consolidated rocks or sand. When this oil is produced through the well bore, it is relatively free of any rock particles or sand. However, many oil and gas wells produce fluid from underground formations which are not consolidated. That is, the various sand particles are not strongly attached to each other and when the fluid is produced it carries entrained sand with it. This sand causes serious damage to well equipment. In producing oil and gas from such unconsolidated formations some method must be provided to restrain the said inflow into the well. There are four primary methods previously in use for this. These are:
1. Consolidation of the formation with plastic binding agent. PA1 2. Placing a screen in the well with sufficiently small openings to prevent inflow of all undesired material. PA1 3. Use of a gravel pack placed in the annular area between a screen and the formation; the voids between the gravel grains being small enough to prevent inflow. PA1 4. Placing a screen in a well consisting of a mechanical screen and a preformed "gravel pack" attached to the screen.
All of these methods, with the exception of the first one, have one common characteristic; they restrain material all on one surface. This provides a perfect environment for plugging, i.e., coarse and fine materials are restrained together, permitting progressive plugging of the screen until essentially complete plugging occurs.
Method number 1, while desired, is very difficult and in some instances impossible to perform in the field. Screens as described in number 4 above have been utilized in the Tar Sands project of Northern Canada with success, but they are easily plugged during installation. This plugging again all occurs at essentially the outer surface of the liner.
Thus, there is a need for a sand filter which prevents such surface plugging and the tendency to plug during installation.